P0171 Driving me Insane!!!

[Ecomike]

" and left the ignition in the ON position."

I wonder if that is a problem? leaving the ignition on. My Renix freaked out one day when my daughter did that to run the radio for 15 minutes. When I started it the idle ran up to 3500 rpm and held! I had to shut it off and restart it. The high idle never returned.

Not being able to disconnect the battery sucks! It is possible to wire in a memory saver battery!

Thanks for the suggestions, I'll give these ideas a go later in the week. I'll also finish summarizing the last 14 pages and make an updated post here as well as on an LPG forum.


He definitely had some sort of OBD-II scanner connected to the PCM because it was still hooked up when we went for a test drive. I don't know if he was looking at the fuel trims though? He also had some sort of DOS computer which he used to program the LPG ECU, apparently to improve performance & economy (all of which will be reset if I disconnect the battery).

Found an interesting link on LPG systems here:

http://www.cherokeeforum.com/f5/lpg-dual-fuel-systems-65184/

In regards to the P0171 code being set at engine start up this may not be the case. I drove around for 20-30 mins today watching the Live Data with NO FAULT Codes showing and stopped the engine (still with no fault codes showing on the laptop) and left the ignition in the ON position. I then re-connected the OBD-II software because the connection was lost when I turned off the engine. Once the OBD-II software restarted/rescanned the fault suddenly appeared (without starting the motor back up) so it looks like the fault is only detected by the OBD-II software when the connection to the PCM is reset - not when it actually occurs.

 

[XJSteven]

" and left the ignition in the ON position."

I wonder if that is a problem? leaving the ignition on. My Renix freaked out one day when my daughter did that to run the radio for 15 minutes. When I started it the idle ran up to 3500 rpm and held! I had to shut it off and restart it. The high idle never returned.

Not being able to disconnect the battery sucks! It is possible to wire in a memory saver battery!

No, leaving the ignition ON wasn't the problem because I did a slightly different test today...I went for another drive with no FAULT CODES showing and instead of turning the motor off (and losing the OBD-II connection) I let it idle with the software connected. I then manually clicked on "Rescan Vehicle" while still at idle and the P0171 code appeared. So it looks like a problem with the software itself. I guess I could keep clicking on "Rescan Vehicle" while I'm driving along to see when it throws the code but that isn't ideal.

I'll look into the memory saver battery idea. Thanks.

 

[XJSteven]

I spoke to the LPG repairer about the fuel trims today and it looks like he agrees with you guys. His suggestion is to run it on petrol and get that looking right first before trying LPG. He also said to leave my laptop unplugged because with that connected the PCM is in 'diagnostic mode' and can't learn.

 

[XJSteven]

UPDATE

After driving around on petrol for a few days (and also swapping over the throttle body & MAP sensor) I checked the Fuel Trims on petrol yesterday and they were much better, +3% at idle and up over +8% at higher revs, but then when I switched back over to LPG the +/-32% fuel trims figures remained.

I've got an 8 page "LPG install guide" from the manufacturer now (in PDF format) but I'm not sure how to attach it to this post.

 

[Ecomike]

I frequently forget to mention that people need to check for exhaust leaks, cracked exhaust pipes. The O2 sensor compares the exhaust gas O2 concentration to out door ambient air O2 concentrations, and if exhaust gas is blowing on the O2 sensor from a crack or gasket leak, it can throw off the measurements, and the computer will get miss led and waist fuel!!!!!!!!

 

[four_shot]

any updates?

 

[XJSteven]

any updates?

Hi four_shot, still not solved but I'll provide an update at the end of the week.

 

[BrokenSockets]

Been following this thread for a while, quite the diagnostic session. I can only imagine your frustration level with the dual fuel system at this point. I bet if you installed a full electric conversion the fuel trim problem would be gone

 

[four_shot]

Of course...bail on the internal combustion engine all together. Why didn't I think of that!:doh:

seriously though I'm kinda wondering what happened.

 

[XJSteven]

I recently replaced the spark plugs and also replaced the brake shoes because someone on an LPG forum said 'sticky brakes' can cause a sudden change in fuel consumption. Didn't work though - the fuel trim data remains the same and I'm using 33% more LPG than I should be.

I'll be calling the LPG repairer again today to tell him nothing he has said or done has made any difference to my problem.

 

[XJSteven]

The latest info I'm getting from the LPG forum gurus is that my LPG system does not use Fuel Trim data from the PCM. Apparently my LPG ECU is wired directly to the 02 sensor and TPS. If it picks up a lean reading at the 02 sensor it will adjust the LPG air/mixture at the stepper motor. The petrol/gas trims have no bearing on the LPG mixture.

Depending on how my system is wired, the PCM is either seeing real data from the 02 sensor (as a lean condition from the LPG mixture) or it is been fed a fake signal from the LPG ECU which it is no longer accepting for some reason and reading it as lean (which tells me nothing about how the LPG is actually running).

I might go back to running on petrol for a while to confirm my fuel economy is only poor on LPG and not petrol too.

If the LPG system doesn't use the PCM fuel trim data I'm not sure why the latest LPG repairer told me I need to get it running right on petrol before it will run right on LPG. Sounds to me like the LPG mixture is adjusted independently? Why is this so confusing!!!

 

[XJSteven]

Right now may be a good time to write an update summary of where we are, starting from ground zero, year, make model, dual fuel history and so on, for those who may find reading or reading the entire thread too long and confusing now?

Hopefully Steven can write a new complete update summary here soon. I have always been curious about alternate fuel feeds!!!

Considering it's been 8 months and 1 mechanic and 2 LPG repairers haven't been able to fix the problem, it's unlikely I'll find an online solution but here's a summary anyway (just in case I've missed something obvious):

Vehicle: Jeep Cherokee 96 XJ, auto, 4.0L 6 cylinder.

LPG system: SPRINT mixer type system with AEB-297 LPG ECU, installed 24 months ago.

Problem:
LPG fuel consumption - using 33% more LPG than before.
Engine code - P0171 "too lean".
Fuel trims (on LPG) - max +33% with the transmission in "Drive" or above 2500rpm in "Park". In "Park" and below 2500rpm the fuel trims are -33%.
Fuel trims (on petrol) - LTFT +4.69% or +5.47% driving around (STFT approx +2% or +3%).
Note - the Fuel Trim data when running LPG is real data from the 02 sensor and not a simulated signal from the LPG ECU. Petrol fuel trims were higher but have recently improved.

What's NOT causing the problem:
Air filter (replaced)
Spark plugs (replaced)
Spark plug leads (replaced)
Ignition coil (replaced)
Distributor cap & rotor (replaced)
Vacuum leak (smoke test all good, new CCV valves/hoses installed)
Oxygen sensors – upstream & downstream (replaced)
TPS (replaced)
IAC valve (replaced)
MAP sensor (replaced)
Throttle Body (replaced)
Exhaust leak (new headers & manifold gasket installed)
Engine Coolant Temp sensor (voltage test)
AC ripple from the Alternator (checked)
Intake Manifold Air Temp sensor (voltage test)
Battery & Alternator (voltage test)
Bad earth in engine bay (checked & cleaned)
Wiring problem – PCM to 02 sensor, TPS, MAP sensor, ECT sensor, IAC (checked continuity)
Sticky brakes (brake shoes replaced & adjusted)
LPG stepper motor (replaced)
Petrol fuel pump (replaced)
Vacuum gauge test - appears normal.

The motor isn't running rough, seems to be running normal on both fuels. I'm in the process of retesting the fuel economy on petrol but I think the P0171 code is only triggered on LPG.

 

[gradon]

You still haven't explained how the injectors are interrupted when on LPG. Maybe there's no interruption and instead, the LPG system assumes the pcm will pull out fuel, which it looks to do by 33%(max pcm can compensate) at idle, but once you start driving, the LPG isn't rich enough, so the PCM swings to the opposite extreme cause it's too lean(sets the code). Do you notice gas consumption while on LPG? Maybe the whole problem is that the injectors are interrupted and because the pcm tries to compensate, but isn't allowed and nothing happens, it takes it to the max in both directions. I don't know what AFRs the LPG looks for, but maybe you should try tuning the idle fuel trim to zero first, then adjust the stepper motor in the higher rpms until the FTs look acceptable(your petrol %s are fine). If the bottom line is that it works on both systems but you have a CEL due to the LPG side, so be it.

 

[four_shot]

Judging by the fuel trim #s on petrol, the problem is in the lpg system. My $.02 is that the LPG sys isn't fully tricking the OBDII system into thinking that it's still in full operation. So the factory pcm is trying to control the fuel system even though the LPG is in operation (inducing a false P0171). I'd be curious to see how the LPG ECM taps into the Factory PCM. I'm starting to get the feeling the problem is there.

Or I could be tired and frusterated and looking to beat my head against a brick wall :banghead:

 

[XJSteven]

You still haven't explained how the injectors are interrupted when on LPG. Maybe there's no interruption and instead, the LPG system assumes the pcm will pull out fuel, which it looks to do by 33%(max pcm can compensate) at idle, but once you start driving, the LPG isn't rich enough, so the PCM swings to the opposite extreme cause it's too lean(sets the code).

Next to the LPG ECU I found a little black box called an Injector Emulator. A quick search shows this device switches off the injectors and emulates the fuel injectors pulse to the PCM, fooling it into thinking the injectors are still working as normal. Apparently it uses resistors to reduce the Amps on the injector wiring so the injector can't open up. The Emulator prevents the PCM from throwing up an error code saying there is a problem with the injectors not working.

Do you notice gas consumption while on LPG?

No, I haven't noticed any gas consumption while running LPG. Should I be able to unplug an injector while running LPG to check there is no fuel there (and confirm the Injector Emulator is working)?

Maybe the whole problem is that the injectors are interrupted and because the pcm tries to compensate, but isn't allowed and nothing happens, it takes it to the max in both directions. I don't know what AFRs the LPG looks for, but maybe you should try tuning the idle fuel trim to zero first, then adjust the stepper motor in the higher rpms until the FTs look acceptable(your petrol %s are fine). If the bottom line is that it works on both systems but you have a CEL due to the LPG side, so be it.

Thanks for posting this Gradon, it's starting to make sense now! I posted this on an LPG forum and here is the reply:

"Spot on. If the LPG is running very slightly lean the PCM increases the time the petrol injectors open for to correct what it sees as an error. This has no effect on the output of the O2 sensor so it ads a bit more which still has no effect, so it adds a bit more, and so on until it gets so far out of what it considers to be normal that it triggers the light. The light is saying, you're running lean but no matter what I do, I can't correct it.."

Except in my case the 33% increase in the cost to fill my LPG tank is more than "slight".

I guess this would mean my Fuel Trims will always be maxed out on LPG. If even the slightest rich or lean reading will do it, what are the chances of the Fuel Trims staying perfectly on 0%? It seems to me that even when my car is running normal on LPG I will still see large fuel trims and probably a "too lean" or "too rich" code so I should completely disregard this data in future...

I still need to work out the underlying problem with the LPG mixture so I might mention your tuning idea to an LPG repairer. I'm not sure about the AFR's but the LPG ECU is supposed to adjust the mixture at the stepper motor using data from the 02 sensor.

 

[XJSteven]

I'd be curious to see how the LPG ECM taps into the Factory PCM. I'm starting to get the feeling the problem is there.

According to a new wiring diagram the LPG manufacturer emailed me yesterday the LPG ECM taps into the following wires at the PCM:

1. PIN A24 on black plug - black/green wire (oxygen sensor)
2. PIN A7 on black plug - grey wire (to negative coil)
3. PIN A23 on black plug - orange/blue wire (TPS).

And that's all there is to it!

 

[Ecomike]

Been out sick for the last 10 days, still not well, but just took a quick look here.

Have we discussed the possibility that the O2 sensor wire data feed to the LPG ECM may have a bad connection, as in added resistance that is biasing the signal voltage the LPG ECU sees, and yet the Petrol, Jeep ECU is getting good data from the O2 sensor?

Such a bad connection could be at a connector, male/female, at the harness of the ECM main connector, or even in the ECM circuit board, like a weak solder joint of the LPG's ECM!

The data says the LPG ECU is seeing a little richer O2 data than the Jeep OEM PCM is seeing, or the LPG ECM is not able to control the LPG feed rate!

 

[four_shot]

I was looking at the wiring diagram from the lpg manufacturer, and the lpg ecu is in series with the o2 sensor. If it were a resistance issue, I would think that it would show up on petrol as well.

I'm leaning twards the stepper motor not opening the valve enough and leaning out the system at cruise speeds. It may be just an adjustment issue?

There is the off possibility that the LPG ECU is faulty.

The most frusterating thing for me is that its halfway around the world and I can't look at the d@mned thing myself...lol!

 

[Ecomike]

Series? They should be in parallel, the ECM/PCM, and should be monitoring the output voltage seperately from the O2 sensor, using high impedance circuits, I would think. Not even sure how a series set up for that work?

I was looking at the wiring diagram from the lpg manufacturer, and the lpg ecu is in series with the o2 sensor. If it were a resistance issue, I would think that it would show up on petrol as well.

I'm leaning twards the stepper motor not opening the valve enough and leaning out the system at cruise speeds. It may be just an adjustment issue?

There is the off possibility that the LPG ECU is faulty.

The most frusterating thing for me is that its halfway around the world and I can't look at the d@mned thing myself...lol!

 

[four_shot]

I'm wondering if the LPG ECM doesn't spoof an o2 sensor signal during LPG operation to prevent unwarranted DTCs. That would explain the need to run the LPG electronics in series.

An internal disconnect type of system? (signal passes through in petrol mode. In LPG, it is interrupted and a dummy signal gets sent to the Petrol pcm to keep it happy.) :dunno: